This application claims the benefit of the European Patent application 02 002 465.9 filed Feb. 1, 2002.
1. Field of the Invention
The invention relates to an apparatus for performing a pull test, to determine tensile strength of a bonding wire connection tensile strength.
2. Description of the Related Art
The pull test has long been the most commonly used method of testing bonding wire connections. In this method a small tension hook is inserted under a loop of wire between two bonding contacts and is moved away with constant velocity in a direction perpendicular to the surface of the substrate (i.e., usually upwards) until the loop breaks or a predetermined force has been reached. The tensile force applied at each moment is determined with a force-measurement device, and the value of tensile force that is being applied when the wire loop breaks away is recorded as the pull-off strength, which provides information about the quality of the bonding connection.
With this procedure, for instance, weakening of the wire in the heel region can be very well detected, as can the quality of the welding. The results of the test are compared with the minimal tensile strength for bonding wire connections specified in the relevant quality publications (e.g., MIL-STD883 method 2011).
Known test apparatus for carrying out this procedure, in which the force measurement is customarily accomplished by detecting the deformation the tensile force produces in an extension arm to which the hook is attached, have proved to be disadvantageous in certain respects. The precision of the measurements seems on the whole to be in need of improvement, and problems have arisen regarding reproducibility under conditions of changing ambient temperature.
The object of the invention is thus to make available an improved test apparatus of this generic kind, which in particular is designed to provide more accurate and reproducible test results even if the ambient temperature is variable.
This object is achieved by a test apparatus including a drive rod with tension hook positioned at an end thereof that can be controllably inserted under the wire loop, a drive device that is connected to the tension hook and generates a tensile force alone a tensile force vector (F) directed substantially perpendicular to the substrate surface, a force-measurement device comprising at least one load cell disposed coaxially with the drive rod and associated with the tension hook to detect the tensile force at each moment, a recording device engaged with the force-measurement device in order to record a pull-strength value for the bonded-wire connection, wherein the force-measurement device is disposed substantially coaxial with the tensile force vector (F), a detector to detect the highest point of the bonding wire loop, and a position controller adapted for automatic positioning of the tension hook and hence the point of origin of the tensile force vector; below the highest point of the bonding wire loop.
The invention includes the idea of departing from the previously customary extension-arm principle and instead disposing the force-measurement device so that it is substantially coaxial with the tensile force vector generated by the drive unit. It further includes the idea of ensuring that the wire loop is reliably grasped at its highest point, so as to avoid errors in the measurement results introduced by force components deviating in direction from the perpendicular to the substrate surface. For this purpose, in accordance with the invention, detection means to detect the highest point in the bonding wire loop and position controllers for automatic positioning of the tension hook in such a way that the starting point of the tensil force vector is below the highest point of the wire loop are position are provided.
The force-measurement device preferably comprises at least one load cell so disposed as to be coaxial with a drive rod of the tension hook. In the interest of the extensive temperature compensation, the force-measurement device preferably comprises a combination (a pair) of load cells disposed one above the other.
In another preferred embodiment an air bearing of the drive rod of the hook is provided, so as for practical purposes to eliminate the frictional forces that are produced in conventional bearings and that reduce the accuracy of the measurements.
In still another advantageous development of the idea behind the invention, a motor-gearbox unit is provided to rotate the tension hook about an axis aligned with the direction of action of the drive device, i.e. to fix the hook at a predetermined angle with respect to the tensile force vector. This measure serves on one hand to facilitate manipulation of the test apparatus in the case of substrates with complex bonding geometry, and on the other hand contributes towards the above-mentioned goal of increasing measurement accuracy, because it makes it possible to avoid potentially error-introducing positions of the hook relative to the wire loop.
Achievement of both of the above-mentioned advantages is also assisted by the provision of an x-y table for the coordinate-controlled positioning of the substrate (and hence of the highest point of the loop that the hook is meant to engage) with reference to the hook.
This measure should be regarded as closely linked to the provision of a camera with a field of view directed towards the side of the wire loop, and an image evaluation device connected to the camera, with which to calculate the coordinates of the highest point in the wire loop in the camera image. Alternatively, it can also be combined with the provision of a movement or proximity sensor associated with the drive unit, in particular the drive rod of the tension hook, and of a control and evaluation unit associated therewith. The latter serves to detect the maximal value of a plurality of positions at which the hook can engage the bonding-wire loop, and to determine the x-y coordinates of the engagement position at which the height is maximal. It will be clearly evident that the result of the measurement of the position of the highest point in the wire loop can be taken as a starting point for a correspondingly calculated shifting of the position of the sample table.
In another advantageous embodiment of the invention, spring means are disposed in association with the hook and the drive unit, in order to prestress the hook into a specified initial position, and also to attenuate the engagement between hook and wire loop and/or to limit the amount of tensile force, in order to protect the force-measurement device. In particular, the spring means comprise at least one first spring with low spring constant, for prestressing into the initial position and attenuate the engagement, and a second spring with high spring constant to limit the tensile force, both springs preferably being disposed coaxially with the drive rod. This arrangement results in a construction that is simple to manufacture and to maintain, while leverage and moments of tilt that might introduce errors into the measurement results are fundamentally ruled out.